The invention relates to a method and an apparatus for holding and/or conveying platelike substrates, particularly wafers, by means of handling systems in inspection and production machines, particularly in wafer inspection devices, wherein a movable arm of the handling system comprises a gripper to which a wafer to be conveyed can be secured through supporting elements on the gripper, and means are provided for securing to the gripper.
In addition, the invention relates to a method and an apparatus for the contactless recognition of platelike substrates to be held and/or to be conveyed, particularly wafers.
When processing and/or inspection is to be carried out, such a substrate or wafer generally has to be removed from storage in a so-called wafer cassette, be temporarily placed on a support of a processing and/or inspection table and, after appropriate processing and/or inspection, be finally stored in a further wafer cassette.
During the processing and/or inspection operation, the wafers to be processed and/or inspected are in a special atmosphere so as to be able to prevent any kind of contamination, and furthermore, during the conveying, the position once taken up in the conveying system must be prevented from changing in an uncontrolled manner.
New submicrometre technologies not only require the fronts of substrates and wafers to be absolutely free of contamination but also the backs to be free of particles.
For example, even contamination of the backs of wafers can lead to defocussing of a micro-lithographic exposure. Thus it is a standard requirement on many substrates, such as glass substrates for masks for example, that they must be absolutely free of contamination on both sides.
In addition, secure and reliable automatic grasping of the substrate or of the wafer by the corresponding gripper is an essential requirement of a wafer processing and/or inspection device working at high speed.
A first example of such a gripper arrangement is known from German Patent Publication 3917260. In this a gripper is provided with a pick-up device which contains a vacuum device, in which case the wafers to be inspected are drawn against the pick-up device over a portion of the platelike surface via suction holes and can thus be conveyed. With such an arrangement, the wafer is attracted by suction applied at least partially to one of its most sensitive surfaces and so contamination cannot be prevented there. In addition, the wafers or substrates that require to be handled are extremely thin discs and are not of a material which is very resistant to bending; as a result of the unilateral loading which occurs, for example, during swivelling movements with the wafer thus gripped, occasional breakage during handling has to be accepted in consequence. This is an unacceptable disadvantage in such vacuum systems.
Another disadvantage which cannot be ignored in the case of substrates held under vacuum is the fact that in the event of failure of the vacuum, the substrate falls down from the gripping location and is liable to be damaged, as is also the case if, with substrates held by suction at one side, a drop in the vacuum force occurs accidentally or as a result of the article slipping sideways--that is to say it loses its position--on swivelling movements of the gripper.
An alternative gripper arrangement is known from German Patent Publication 3402664, using two flexibly guided supporting elements but this is unsatisfactory if only because of the nature of the movements to be executed. The use of belt drives as proposed in this case, leads to such a great risk of contamination and instability as well as fluctuations during movements, that system vibrations would be a direct consequence.
Furthermore, in this arrangement a further disadvantage arises with regard to the detection of a wafer to be gripped. Detection is only assured to the extent that the coordinate system at the beginning of the inspection or processing can be kept stable since it is customary to operate robotically from such a coordinate system. Even if precautions are taken to mount the inspection device isolated from vibrations and oscillation, the accuracy and reliability which can be achieved in this manner cannot be regarded as adequate.
Instead, care must be taken to ensure that, with such handling systems, before a wafer to be conveyed or processed is grasped, the movement can be carried out each time in a controlled manner, after renewed adjustment, particularly in view of the fact that, in the storage state, the wafers are generally in standard cassettes which only provide very little spacing between two adjacent wafers.
Not least, the system should be able to detect whether a wafer is or is not present for inspection. For contactless detection of objects over relatively great distances, optical systems inter alia have gained acceptance for the purpose, particularly in two kinds, namely (i) transmitter and receiver of a light source are arranged so that the object to be detected comes to lie between transmitter and receiver, or (ii) the light from the transmitter reflected or scattered by the object is detected by a receiver.
Systems of the first kind generally work very reliably but have the disadvantage that transmitter and receiver have to be spatially separated from one another constructionally which thus makes a compact form of structure impossible. Systems of the second kind are more unreliable because the condition and geometry of the surface exert a great influence. On the other hand, they permit movement of the transmitter, which is often an advantage.